The invention relates to hydraulic anti-vibratory devices intended to be inserted for support and shock-absorbing purposes between two rigid elements individually subject to certain oscillations or vibrations, said elements forming, for example, part, respectively of a vehicle chassis and of the internal combustion engine or of a suspension train of this vehicle.
The invention relates more particularly, among supports of the type concerned, to those which are constituted by a sealed box comprising a rigid core which can be made fast with one of the two rigid elements, a rigid base which can be made fast with the other rigid element, an annular elastic support wall, preferably of frustoconic shape, sealably connecting the core to the base, the inner space of said box being filled with a shock-absorbing liquid and the core being extended, on the inside of the box, by a rigid foot itself terminated by a shoe or piston whose contour defines, with the opposite wall portion of the box, a throttled annular passage for the liquid.
In supports of this type, the oscillations applied axially to the core or to the base are manifested by alternate back-thrusts of the piston in the liquid and, for a well-determined value F.sub.O of the alternation frequency of these back-thrusts, the annular column of liquid which occupies the throttled annular passage surrounding the piston is subjected to a resonance phenomenon which ensures excellent damping of the oscillations concerned.
The drawback of such supports resides in the value F.sub.O being imposed by the dimensions of the annular passage and hence difficult to modify.
Now it is often advantageous to be able to adapt supports of the type concerned to the preferential damping of oscillations whose frequencies have values F.sub.1, F.sub.2. . . a little different from F.sub.O, and to do this for given sizes of the annular passage, that is to say without it being necessary to replace the piston or the box in which this piston is immersed.
It is an object of the present invention to provide a particularly simple formula to resolve such a problem.